Implantable medical devices and in particular heart stimulators can be used for treating a variety of heart disorders like bradycardia, tachycardia or fibrillation by way of electric stimulation pulses delivered to the heart tissue, the myocardium. The ability of such devices to pick-up electrical potential in a heart is often times used to acquire an intracardiac electrogram (IEGM) that is a heart signal representing the time course of the electric potential picked up by the implant.
Depending on the disorder to be treated, such heart stimulator generates electrical stimulation pulses that are delivered to the heart tissue (myocardium) of a respective heart chamber according to an adequate timing regime. Delivery of stimulation pulses to the myocardium is usually achieved by means of an electrode lead that is electrically connected to a stimulation pulse generator inside a heart stimulator's housing and that carries a stimulation electrode in the region of its distal end. A stimulation pulse also is called a pace. Similarly, pacing a heart chamber means stimulating a heart chamber by delivery of a stimulation pulse.
In order to be able to sense a contraction a heart chamber that naturally occurs without artificial stimulation and that is called intrinsic, the heart stimulator usually comprises at least one sensing stage that is connected to a sensing electrode on said electrode placed in the heart chamber. An intrinsic excitation of a heart chamber results in characteristic electrical potentials that are picked up via the sensing electrode and that can be evaluated by the sensing stage in order to determine whether an intrinsic excitation—called: intrinsic event—has occurred.
Usually, a heart stimulator features separate stimulation generators for each heart chamber to be stimulated. Therefore, in a dual chamber pacemaker, usually an atrial and a ventricular stimulation pulse generator for generating atrial and ventricular stimulation pulses are provided. Delivery of an atrial or a ventricular stimulation pulse causing an artificial excitation of the atrium or the ventricle, respectively, is called an atrial stimulation event AP (atrial paced event) or a ventricular stimulation event VP (ventricular paced event), respectively. The strength of stimulation pulses delivered by the respective stimulation pulse generator is adjustable in order to be able to adjust the stimulation pulse strength to be just sufficient to cause capture (above capture threshold) and thus using as little energy as possible to be effective. Stimulation pulse strength depends on both, duration and amplitude of the stimulation pulse.
Common heart stimulators feature separate sensing stages for each heart chamber to be of interest. In a dual chamber pacemaker usually two separate sensing stages, an atrial sensing stage and a ventricular sensing stage, are provided that are capable to detect intrinsic atrial events AS (atrial sensed event) or intrinsic ventricular events VS (ventricular sensed event), respectively.
As known in the art, separate sensing and pacing stages are provided for three-chamber (right atrium RA, right ventricle RV, left ventricle LV) or four-chamber (right atrium RA, left atrium LA, right ventricle RV, left ventricle LV) pacemakers or ICDs.
By means of a sensing stage for a heart chamber to be stimulated, the pacemaker is able to only trigger stimulation pulses when needed that is when no intrinsic excitation of the heart chamber occurs in time. Such mode of pacing a heart chamber is called demand mode. In the demand mode the pacemaker schedules an atrial or a ventricular escape interval that causes triggering of an atrial or ventricular stimulation pulse when the escape interval times out. Otherwise, if an intrinsic atrial or ventricular event is detected prior to time out of the respective atrial or ventricular escape interval, triggering of the atrial or ventricular stimulation pulse is inhibited. Such intrinsic (natural, non-stimulated) excitation are manifested by the occurrence of recognizable electrical signals that accompany the depolarization or excitation of a cardiac muscle tissue (myocardium). The depolarization of the myocardium is usually immediately followed by a cardiac contraction. For the purpose of the present application, depolarization and contraction may be considered as simultaneous events and the terms “depolarization” and “contraction” are used herein as synonyms. The recognizable electrical signals that accompany the depolarization or excitation of a heart chamber are picked up (sensed) by the atrial or the ventricular sensing channel, respectively. Thus, by means of the sensing stages, intracardiac electrogram signals are acquired, that can be evaluated by the implantable medical device. Simple evaluation only checks whether the IEGM exceeds a given threshold in order to detect a sense event. More complex evaluation includes analysis of the IEGM's morphology.
In order to allow for such morphology analysis, it is desirable to record the time course of an IEGM signal by means of sampling the signal. Sampling is carried out by measuring the signals amplitude at predetermined points of time with a constant sampling interval.
In a heart cycle, an excitation of the myocardium leads to depolarization of the myocardium that causes a contraction of the heart chamber. If the myocardium is fully depolarized it is unsusceptible for further excitation and thus refractory. Thereafter, the myocardium repolarizes and thus relaxes and the heart chamber is expanding again. In a typical electrogram (EGM) depolarization of the ventricle corresponds to a signal known as “R-wave”. The repolarization of the ventricular myocardium coincides with a signal known as “T-wave”. Atrial depolarization is manifested by a signal known as “P-wave”. For evaluation of an IEGM it is desirable to be able to determine these particular signals.
With respect to capture control, it is further desirable to have a representation of an IEGM that allows for discrimination between a polarization artifact following an ineffective stimulation pulse and an evoked response following an effective stimulation pulse.
Several modes of operation are available in a state of the art multi mode pacemaker. The pacing modes of a pacemaker, both single and dual or more chamber pacemakers, are classified by type according to a three letter code. In such code, the first letter identifies the chamber of the heart that is paced (i.e., that chamber where a stimulation pulse is delivered), with a “V” indicating the ventricle, an “A” indicating the atrium, and a “D” indicating both the atrium and ventricle. The second letter of the code identifies the chamber wherein cardiac activity is sensed, using the same letters, and wherein an “O” indicates no sensing occurs. The third letter of the code identifies the action or response that is taken by the pacemaker. In general, three types of action or responses are recognized: (1) an Inhibiting (“I”) response wherein a stimulation pulse is delivered to the designated chamber at the conclusion of the appropriate escape interval unless cardiac activity is sensed during the escape interval, in which case the stimulation pulse is inhibited; (2) a Trigger (“T”) response wherein a stimulation pulse to a prescribed chamber of the heart a prescribed period of time after a sensed event; or (3) a Dual (“D”) response wherein both the Inhibiting mode and Trigger mode may be evoked, e.g., with the “inhibiting” occurring in one chamber of the heart and the “triggering” in the other.
To such three letter code, a fourth letter “R” may be added to designate a rate-responsive pacemaker and/or whether the rate-responsive features of such a rate-responsive pacemaker are enabled (“O” typically being used to designate that rate-responsive operation has been disabled). A rate-responsive pacemaker is one wherein a specified parameter or combination of parameters, such as physical activity, the amount of oxygen in the blood, the temperature of the blood, etc., is sensed with an appropriate sensor and is used as a physiological indicator of what the pacing rate should be. When enabled, such rate-responsive pacemaker thus provides stimulation pulses that best meet the physiological demands of the patient.
A dual chamber pacemaker featuring an atrial and a ventricular sensing stage and an atrial and a ventricular stimulation pulse generator can be operated in a number of stimulation modes like VVI, wherein atrial sense events are ignored and no atrial stimulation pulses are generated, but only ventricular stimulation pulses are delivered in a demand mode, AAI, wherein ventricular sense events are ignored and no ventricular stimulation pulses are generated, but only atrial stimulation pulses are delivered in a demand mode, or DDD, wherein both, atrial and ventricular stimulation pulses are delivered in a demand mode. In such DDD mode of pacing, ventricular stimulation pulses can be generated in synchrony with sensed intrinsic atrial events and thus in synchrony with an intrinsic atrial rate, wherein a ventricular stimulation pulse is scheduled to follow an intrinsic atrial contraction after an appropriate atrioventricular delay (AV-delay; AVD), thereby maintaining the hemodynamic benefit of atrioventricular synchrony.
From the foregoing it becomes apparent that there is a need to provide the physician with a graphical representation of an intracardiac electrogram in order to facilitate heart diagnosis and optimize the mode of pacemaker operation.
The IEGM acquired by the implantable medical device can either be stored in the implant itself or be telemetrically transmitted to a central service center remote from the individual implant. For both cases it is preferred to have as little data as possible to be stored or transmitted. Therefore there is a general need for an effective data representation of a time course of a signal such as an IEGM.
A method for an effective representation of an IEGM is known from U.S. Pat. No. 5,836,889. U.S. Pat. No. 5,836,889 discloses a method and apparatus that identifies turning points in an intracardiac EGM that is sampled at equidistant time points by comparing the slope between an actual sample (n) value and the second last sample (n−1) value with the slope between the actual sample (n) value and the last identified turning point. If the difference between the two slopes thus determined exceeds a predetermined threshold, the second last sample value is marked as a further turning point of the EGM. Once all turning points are thus identified, only the turning points of the EGM signal are stored as a compressed data representation of the EGM signal whereas those sample values not being identified as turning points can be discarded. The slope is determined by determining the difference quotient between two samples. The difference quotient for an actual sample and the second last sample approximately corresponds to the first derivative with respect to time of the EGM signal, because the actual sample and the second last sample are immediate neighbours. The disclosure of U.S. Pat. No. 5,836,889 is included herein by reference.